THE PHYSICAL CHEMISTRY OF SEA WATER 557 



many uncertain factors, it does not seem worth while to strive for 

 greater accuracy in those isolated points where it could be achieved. 

 The present treatment may be adequate for the purpose, which is 

 to provide a basis for discussing the main species present in the 

 solution, and the solid phases that determine the composition of 

 the solution. 



Major Constituents 

 Water and Silica 



In mixing our ocean model, we start with 54.90 mole H2O. The 

 elements will usually be added as hydroxides, or oxides. Since HoO 

 is present in great excess, we shall not be very particular about 

 counting the O and H atoms. 



First, we add 6.06 mole SiOo. At equilibrium, it would be present 

 as solid quartz, SiOo, an important constituent of marine sedi- 

 ments. The solubility of quartz in water has been given as lO^'^-'^'M 

 at 25°C (lO-'-'^M at 0°C) (Van Licr, 1959). The Si is likely to 

 exist mainly as mononuclear units Si (OH) 4 and not as a colloid as 

 was believed earlier. 



For the bottom water of the ocean, figures from 1000 mg Si/m'^ 

 (Atlantic) to 3000 (Antarctic) have been given, which would 

 correspond to lO"^-^ to 10-^-"M. In the upper layers, much lower 

 concentrations (below 10"^) have been observed, which is 

 ascribed to the action of diatoms. 



The acidity constants of Si (OH) 4 in 0.5M NaCl at 25°C are 

 10-«-^i and 10-i--'i (Ingri, 1959). At the pH of the ocean, only 

 about 5 per cent of the Si would then exist as SiO(OH)3~, or 

 H3Si04-. 



Aluminum 



We return to our imaginary experiment, and add 1.85 mole 

 A1(0H)3 to the mixture. At real equilibrium there would now be 

 two solid phases: one is quartz SiOo, and the other is probably 

 kaolinite, Al2Si205(OH)4. In the solution, the equilibrium concen- 

 tration of Si(0H)4 will be unchanged, since we have quartz in 

 excess, as always in the following. 



